\(a\)  is the scale factor, \(\xi \)  is the non–minimal coupling to gravity [1]} and the prime denotes differentiation with respect to conformal time \(\eta \) . Time variation of \(\omega _k\)  is non–adiabatic if
\(\omega ^\prime _k /\omega _k^2 \gtrsim 1\) , which implies particle creation due to expansion. For low \(k\)  and conformal coupling \(\xi =1/6\) , this is equivalent to \(a^\prime /a^2 = H \gtrsim m_\chi \)  such
that particles lighter than the Hubble rate \(H\)  are constantly created. The effect can be even stronger for non–conformal \(\xi \) . The accumulated abundance of \(\chi \)  can constitute dark matter [2]}, [3]},
depending on \(m_\chi \)  and its self–interaction.
